ABSTRACT: The specific content, dissolved uptake rate, dietary assimilation efficiency (AE), and efflux rate constant (ke) of calcium (Ca) were quantified in juvenile (4-d) and adult (10-d) Daphnia magna cultured in low (0.5 mg L-1)- and high (50 mg L-1)-Ca environments using a radiotracer technique. Daphnids raised in the high-Ca environment had higher Ca contents than did those raised in the low-Ca environment, and juvenile daphnids had higher Ca contents than adults. Uptake from solution was the dominant source (97-100%) of Ca for daphnids. The maximum influx rate (Jmax) was higher in juvenile daphnids (3.24-4.10 mg g dry weight [wt]-1 h-1) than in adults (1.51-1.62 mg g dry wt-1 h-1), while the influx rates were comparable in different Ca environments. The half-saturation concentration (Km) was 2.51-5.58 mg L-1. The AEs of Ca declined exponentially with increases in food concentrations, and lower AE was observed in the higher Ca environment. The ke of Ca (0.83-1.98 d-1) was the highest among the elements whose ke had been quantified in D. magna, and it was 1.8-2.4 times higher in the high-Ca environment. Excretion into water was the dominant route (60-85%) of Ca release from daphnids; another 15-40% of Ca was lost as molts. The regulation of Ca in daphnids is mainly accomplished through adjusting their efflux but not their influx of Ca; their regulation ability is very limited, which may lead to a poor performance in daphnids in Ca-deficient water.

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